+ MCGA in no-EGA/VGA mode #17
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Summary of Changes
Hello @DnCraptor, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!
This pull request significantly improves the emulation accuracy for MCGA (Multi-Color Graphics Array) when a full EGA/VGA card is not present. It addresses how the emulator reports its display capabilities via BIOS calls and refines the handling of I/O port writes to align with the specific characteristics of MCGA hardware.
Highlights
- MCGA Capability Reporting: I've updated the BIOS interrupt 10h, function 1Ah (Get Display Combination Code) handler in
src/emulator/cpu.cto correctly report MCGA capabilities. When EGA/VGA is not enabled, the emulator will now returnBL=0x05to indicate MCGA support, ensuring compatibility with software that queries display types. - Refined VGA/MCGA Port Handling: In
src/emulator/ports.c, I've adjusted the I/O port write logic for VGA/MCGA. Specifically, writes to ports0x3C4and0x3CEare now ignored if EGA/VGA is not enabled, as these ports are not present in MCGA. Other common VGA ports (like0x3C0,0x3C5-3C9,0x3CF) are now always processed by thevga_portoutfunction, regardless of whether full EGA/VGA is enabled, allowing for proper MCGA-specific register access.
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| case 0x3C4: // no such ports in MCGA | ||
| case 0x3CE: | ||
| if (!ega_vga_enabled) break; | ||
| case 0x3C0: | ||
| case 0x3C4: | ||
| case 0x3C5: | ||
| case 0x3C6: | ||
| case 0x3C7: | ||
| case 0x3C8: | ||
| case 0x3C9: | ||
| case 0x3CE: | ||
| case 0x3CF: | ||
| if (ega_vga_enabled) | ||
| return vga_portout(portnum, value); | ||
| return vga_portout(portnum, value); |
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The logic for handling MCGA ports is not entirely correct. MCGA does not have a Sequencer (ports 0x3C4, 0x3C5) or a Graphics Controller (ports 0x3CE, 0x3CF). The current code correctly ignores writes to the index registers 0x3C4 and 0x3CE in MCGA mode, but it incorrectly allows writes to the data registers 0x3C5 and 0x3CF to fall through to vga_portout. This can lead to incorrect emulation behavior. All four of these ports should be ignored when !ega_vga_enabled.
case 0x3C4: // Sequencer registers (not on MCGA)
case 0x3C5:
case 0x3CE: // Graphics Controller registers (not on MCGA)
case 0x3CF:
if (!ega_vga_enabled) {
break;
}
/* fallthrough for EGA/VGA */
case 0x3C0:
case 0x3C5:
case 0x3C6:
case 0x3C7:
case 0x3C8:
case 0x3C9:
case 0x3CF:
return vga_portout(portnum, value);
tested on Space Quest 3 - uses linear VGA mode (MCGA)
tested on SysInfo - shows CGA (EGA/VGA not detected as expected)